To answer all these questions, we would need to understand the fundamentals and outlook of the "horses". We would also need to find out the intrinsic valuation of the "horses". As you would have guessed by now, this is a big topic by itself and there's no straightforward answers to these questions. However, from personal experience - one of the better moments to change your "horses" (if you really need to) is probably around the maximum pessimism in the market; this is the moment when almost everything in the market is tumbling and the tumbling doesn't seem to cease anytime soon. Basically, this is the moment when it "feels" like the end of the world. 

But like what many experienced investors will share, determining the market's bottom is as good as anyone's guess, so don't kick yourself too hard if you don't catch it. The point is we should always do our due diligence as much as possible before any purchase of a "horse" and minimize the need to change the "horse" later, but we should also be flexible enough to change the "horse" when we realize the need to do so. In short, we need to strike a good balance with an open mind. Easier said than done, but lessons have taught me that it is often far much better to ride a strong “horse” than a sick one in the long run. 

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